Energy efficient headlamp

ABSTRACT

A reflector assembly is provided for efficiently collecting and forming the electromagnetic radiation from a radiation source into a desired beam pattern. The reflector assembly uses a smaller non-symmetrical generally convex reflector and a larger non-symmetrical generally concave reflector. The generally convex reflector intercepts a large fraction of light emitted by the source that would otherwise escape without interacting with the assembly and redirects this light onto the generally concave reflector for eventual inclusion in the beam.

RELATED APPLICATION

This is a division of application Ser. No. 08/627,186, filed Apr. 3,1996, now U.S. Pat. No. 5,870,235.

BACKGROUND OF THE INVENTION

This invention relates generally to energy-efficient projectors ofelectromagnetic radiation which collect energy emitted from a sourceand, more particularly, to shallow reflector assemblies that efficientlycollect light emitted from a light source and project it into apredetermined beam pattern.

A parabolic reflector is often used to collect light emitted by a lightsource and form it into an approximately collimated beam. The lightsource is situated approximately at the focus of the parabola and thelight beam formed by the reflector exits through a circular orapproximately rectangular aperture centered about the parabola's axisand travels in a direction generally along this axis. A portion of thelight emerges from this system without ever intersecting the reflectorand hence is called stray light. This stray light component representsan inefficiency in collecting the light from the source. For someapplications, this stray light may also cause unwanted glare.

It is known that increasing the depth of the parabola can increase thecollection efficiency. However, size concerns often prohibit the use ofdeep parabolic reflectors and the incremental efficiency gains achievedby increasing the parabola's depth diminish as the depth increases.

For example, in an automobile headlamp, the size of the optical assemblyis often restricted to a relatively small volume. Because the parabolicreflector is relatively shallow, the light source is located relativelyclose to the reflector's exit aperture. This results in a large portionof the light emitted by the source being radiated from the exit aperturewithout being controlled by the reflector. In a typical headlamp, lessthan 40 percent of the light is collected by the reflector forprojection into the beam.

Further, an automobile headlamp must meet certain glare requirements toavoid blinding oncoming drivers. Typically, a lens having generallydiscrete prismatic regions is located in the aperture to assist inredistributing the beam so that the resulting pattern achieves a certainlateral spread and so that the vertical glare is also controlled.However, this lens adds to the cost and complexity of the headlamp'soptical design.

Accordingly, there is a need for a shallow optical assembly thatefficiently collects the light from a source and forms it into a desiredbeam pattern. The present invention satisfies this needs.

SUMMARY OF THE INVENTION

The present invention is embodied in a reflector assembly thatefficiently collects light emitted by a light source and projects thatlight into a predetermined beam pattern. The reflector assembly uses asmaller non-symmetrical generally convex reflector and a largernon-symmetrical generally concave reflector. The shapes of the tworeflectors are derived using nonimaging optical techniques to achievethe desired beam pattern.

In one embodiment of the present invention, the assembly includes afirst reflective surface having a non-parabolic generally concave shapethat defines an exit aperture and a beam projection axis. Also, thefirst reflective surface is non-symmetric about an axis normal to itssurface. The assembly also includes a second reflective surface having agenerally convex shape. The second reflective surface intercepts atleast a portion of the electromagnetic radiation emitted from thesource, and reflects this portion of the radiation toward the firstreflective surface so that the beam's flux is increased.

In a more detailed feature of the present invention, an intersectionbetween the first reflective surface and a plane perpendicular to thebeam projection axis provides a non-symmetric curve. In another moredetailed feature, a curve formed by a longitudinal or lateralcross-section of the first reflective surface includes at least oneconvex extremum. The convex extremum may be bounded by a concaveextremum on each side, and the distances between the convex extremum andeach concave extremum are different.

In another more detailed feature of the invention, a curve formed by alongitudinal or lateral cross-section of the first reflective surfacehas a concave extremum bounded on each side by a convex extremum.Further, the distances between the concave extremum and each convexextremum may be different.

In another embodiment of the present invention, the assembly includes afirst reflective surface having a generally concave shape that definesan exit aperture and a beam projection axis, and a second reflectivesurface having a generally convex shape that is non-symmetric about anaxis normal to the surface. The second reflective surface intercepts atleast a portion of the electromagnetic radiation emitted from thesource, and reflects this portion of the radiation toward the firstreflective surface so that the beam's flux is increased.

In a more detailed feature of the invention, a non-symmetric curve isformed by an intersection between the second reflective surface and aplane perpendicular to the beam projection axis. Further, a curve formedby a longitudinal or lateral cross-section of the second reflectivesurface may also have convex and concave extrema as discussed above withrespect to the first reflective surface.

Other features and advantages of the present invention should becomeapparent from the following description of the preferred embodiments,taken in conjunction with the accompanying drawings, which illustrate,by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective grid plot of a reflector assembly of the printinvention, having a smaller generally convex reflector and a largergenerally concave reflector.

FIG. 2 is a contour plot of the surface of the generally concavereflector of FIG. 1.

FIG. 3 is another perspective grid plot of the surface of the generallyconcave reflector of FIG. 1

FIG. 4A is a grid plot of the surface of the generally concavereflector, viewed from the far end of the reflector as it is shown inFIG. 1

FIG. 4B is a graph showing lateral cross-sections of the generallyconcave reflector of FIG. 1, taken along lines y=-37.5, 0, and +37.5 ofthe contour plot of FIG. 2.

FIG. 5A is a grid plot of the surface of the generally concavereflector, viewed from the near side of the reflector as it is shown inFIG. 1

FIG. 5B is a graph showing longitudinal cross-sections of the generallyconcave reflector of FIG. 1, taken along lines at z=-60, 0, and +60 ofthe contour plot of FIG. 2.

FIGS. 6A-6G are graphs showing, in enhanced form, one or more extremadeviating from a base curve.

FIG. 7 is a contour plot showing a desired far field low-beam intensitydistribution for an automobile headlamp.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in the exemplary drawings, and particularly in FIG. 1, thepresent invention is embodied in a shallow reflector assembly 10 forproviding efficient collection and projection of light from a lightsource into a desired beam pattern. The assembly includes two reflectivesurfaces 12 and 14.

The reflective surface 12 is a generally convex surface. Among thedistinguishing characteristics of the reflective surface 12 is that itis non-parabolic and non-symmetrical and that it may contain uniquemorphological features such as dimples and/or folds. This surface servesto intercept a large fraction of the light emitted by the source, thatwould otherwise escape without interacting with the optical system 10,and redirects this light onto the surface 14.

The reflective surface 14 is a generally concave surface that defines anaperture 15. Among the distinguishing characteristics of the secondreflective surface is that it is non-parabolic and non-symmetrical, asshown in FIGS. 2 and 3. The second reflective surface may also containunique morphological features such as dimples and folds.

Despite the generally convex geometry of the first reflective surface12, the changes in curvature of this surface result in localizedsections of the reflective surface to occur dimples and/or folds whichare locally concave. Similarly, the changes in curvature of the surface14 result in localized sections of the reflective surface 14 to occurdimples and/or folds which are locally convex. For example, as shown inFIG. 4A and 4B, a lateral plane, or plane parallel to the x-y plane at afixed z value, that intersects the reflective surface 14 forms a curvehaving bumps or extrema corresponding to the dimples and/or folds. (Asillustrated in FIG. 4B, the z-axis is normal to the plane of thediagram.) More specifically, a lateral cross-section of the secondreflective surface at z=-60 results in a curve that changes from alocally convex curvature 16 to a locally concave curvature 18, and backto a locally convex curvature 20. Further, the distance h' between oneconvex extremum 16 and the concave extremum 18 is not necessarily equalto the distance h" between the other convex extremum 20 and the concaveextremum 18. A lateral cross-section at z=60 results in a curve thatexhibits an opposite behavior with its curvature changing from a locallyconcave curvature 22 to a locally convex curvature 24 and back to alocally concave curvature 26. At z=0, the resulting curve 27 exhibitssimilar features, but at a smaller scale as discussed below.

A better understanding of the geometry of the reflective surface 14 canbe had with reference to FIGS. 5A and 5B. A longitudinal plane, or planeparallel to the x-z plane at a fixed y value, that intersects thereflective surface 14 likewise forms a curve having extrema. The curvesshown in FIG. 5B are formed by longitudinal cross-sections of thereflective surface 14 at y=-37.5, 0, and +37.5. In FIG. 5B the y-axis isnormal to the plane of the diagram. Along the curve formed at y=0, thesecond surface may be described by defining a distance r, along an axis28, from the surface to a point P located in space a predetermineddistance above the reflector. That distance r may be characterized byfunctions, f(φ) and f' (-φ) from the normal, which depend upon the angleφ. Thus, the distance is equal to a function of the angle φ measured inthe x-z plane, and as φ increases, f(φ) and f' (-φ) may increase ordecrease. The functions may be non-symmetric so that f(φ) and f' (-φ)may increase at a different rate, may increase at the same rate, or onemay increase while the other decreases. Accordingly, in someembodiments, f(φ) and f' (-φ) will be equal, in other embodiments,however, these functions are not equal. Typically, the changes indistance as φ increases, or -φ increases, are preferably, but notnecessarily, piecewise continuous and smooth. A variety ofnon-symmetrical configurations have been determined to significantlycontribute to the desired beam pattern.

The contemplated cross-sectional curves formed by an intersection of allor part of the reflective surface 14 with either of an x-y or lateralplane, or additionally or alternatively with an x-z or longitudinalplane, are shown in FIGS. 6A-6G. The generally concave surface 14 caninclude a convex extremum 30 located between two concave regions 32 and34 (FIG. 6A). The concave surface may include two generally convexextrema 36 and 38 (FIG. 6B). Alternatively, the concave surface mayinclude a convex extremum 40 adjacent a concave extremum 42 (FIG. 6C).The concave surface may additionally or alternatively include threeconvex extrema 44, 46 and 48 (FIG. 6D). In addition, the concave surfacemay include two convex extrema 50 and 52 in various positionalrelationships with a concave extremum 54 (FIGS. 6E-6G). Although theextrema are shown with respect to a cross-section through the entirereflective surface, it is understood that similar features can be foundon a smaller scale. For example, any portion of a lateral orlongitudinal cross-sectional curve may include any of the profile curvesshown in FIGS. 6A-6G.

Further, as successive slices or cross-sections are taken through thereflective surface 14, the curve shown in FIG. 6A may transition to anyof the curves shown in FIGS. 6A-6G. This is best described with respectto FIG. 4B. The curve at z=60 is analogous to the curve shown in FIG.6A. At successively similar z values, the surface transitions until itforms the curve at z=-60, which is analogous to the curve in FIG. 6B.Similarly, the curves shown in FIG. 5B may transition at successivelydifferent y values. This transitioning effect can occur over the entiresurface, or over small portions of the surface.

The non-symmetrical or irregular shape of the reflector 12 and/orreflector 14 should allow the headlamp 10 to achieve a far field beampattern similar to that shown in FIG. 7, without requiring a prismaticlens over the lamp's aperture 15. Further, the first reflector 12prohibits light from exiting the aperture in an uncontrolled fashion andconsequently serves to control glare.

Due to the disposition of the reflector 12 between the source (notshown) and the exit aperture 15, the collection efficiency of the lightis much increased over what would be possible by using only a singlereflector to collect the light.

The geometry of the two reflectors is optimized to provide the desiredbeam pattern using nonimaging optical design techniques that take intoaccount multiple reflections. For further discussion of the intentionaluse of multiple reflections in nonimaging optical design techniques,see, U.S. Pat. No. 5,237,170 to N. Shatz titled, "Method and Apparatusfor Non-imaging Concentration and Projection of ElectromagneticRadiation."

Although the foregoing discloses the presently preferred embodiments ofthe invention, it is understood that the those skilled in the art maymake various changes to the preferred embodiment shown without departingfrom the scope of the invention. The invention is defined only by thefollowing claims.

I claim:
 1. Apparatus for collecting electromagnetic radiation andprojecting the collected electromagnetic radiation into a beam having apredetermined pattern, comprising:a first reflective surface having anon-parabolic generally concave shape, defining an exit aperture, andbeing non-symmetric about an axis normal to its surface; and a secondreflective surface having a generally convex shape, intercepting atleast a portion of the electromagnetic radiation, and reflecting thisportion of the radiation toward the first reflective surface, whereinflux of the beam is increased.
 2. Apparatus for projecting collectedelectromagnetic radiation as defined in claim 1, wherein an intersectionbetween the first reflective surface at a location thereon and a planeperpendicular to the first reflective surface at said location providesa non-symmetric curve.
 3. Apparatus for projecting collectedelectromagnetic radiation as defined in claim 1, wherein a curve formedby a longitudinal cross-section of the first reflective surface includesa convex extremum.
 4. Apparatus for projecting collected electromagneticradiation as defined in claim 3, wherein the convex extremum is boundedon each side by concave extremum.
 5. Apparatus for projecting collectedelectromagnetic radiation as defined in claim 4, wherein the distancesbetween the convex extremum and each concave extremum are different. 6.Apparatus for projecting collected electromagnetic radiation as definedin claim 1, wherein a curve formed by a longitudinal cross-section ofthe first reflective surface has a concave extremum bounded on each sideby convex extrema.
 7. Apparatus for projecting collected electromagneticradiation as defined in claim 6, wherein the distances between theconcave extremum and each convex extremum are different.
 8. Apparatusfor projecting collected electromagnetic radiation as defined in claim1, wherein a curve formed by a longitudinal cross-section of the firstreflective surface includes more than one convex extremum.
 9. Apparatusfor projecting collected electromagnetic radiation as defined in claim1, wherein a curve formed by a longitudinal cross-section of the firstreflective surface includes a convex extremum and a concave extremum.10. Apparatus for projecting collected electromagnetic radiation asdefined in claim 1, wherein a curve formed by a longitudinalcross-section of the first reflective surface includes a convex extremumbounded on one side by a convex extremum and bounded on its other sideby a concave extremum.
 11. Apparatus for projecting collectedelectromagnetic radiation as defined in claim 1, wherein a curve formedby a lateral cross-section of the first reflective surface includes aconvex extremum.
 12. Apparatus for projecting collected electromagneticradiation as defined in claim 11, wherein the convex extremum is boundedon each side by concave extrema.
 13. Apparatus for projecting collectedelectromagnetic radiation as defined in claim 13, wherein the distancesbetween the convex extremum and each concave extremum are different. 14.Apparatus for projecting collected electromagnetic radiation as definedin claim 1, wherein a curve formed by a lateral cross-section of thefirst reflective surface has a concave extremum bounded on each side byconvex extremum.
 15. Apparatus for projecting collected electromagneticradiation as defined in claim 14, wherein the distances between theconcave extremum and each convex extremum are different.
 16. Apparatusfor projecting collected electromagnetic radiation as defined in claim1, wherein a curve formed by a lateral cross-section of the firstreflective surface includes more than one convex extremum.
 17. Apparatusfor projecting collected electromagnetic radiation as defined in claim1, wherein a curve formed by a lateral cross-section of the firstreflective surface includes a convex extremum and a concave extremum.18. Apparatus for projecting collected electromagnetic radiation asdefined in claim 1, wherein a curve formed by a lateral cross-section ofthe first reflective surface includes a convex extremum bounded on oneside by a convex extremum and bounded on its other side by a concaveextremum.
 19. Apparatus for projecting collected electromagneticradiation as defined in claim 1, wherein the first reflective surface ispiecewise continuous.
 20. A headlamp projecting electromagneticradiation into a beam having a predetermined pattern, comprising:alarger reflective surface defining an exit aperture, the largerreflective surface having a generally concave configuration; and asmaller reflective surface position between the larger reflectivesurface and the exit aperture thereof, the smaller reflective surfacehaving a generally convex configuration and being non-symmetric about anaxis normal to its surface, the smaller reflective surface interceptingat least a portion of the electromagnetic radiation that is not radiateddirectly toward the larger reflective surface, and reflecting thisportion of the radiation toward the larger reflective surface, whereinthe reflective surfaces are of a predetermined configuration and theflux of the beam is increased.